Elements 11, Face Recognition

I try not to spend Other People's Money. But, if you have a lot of faces to tag, I strongly recommend that you upgrade to the latest version of Elements, if you are able to do so.   I just took a look at Elements 11 face recognition and it is very rudimentary compared to the current version.  It doesn't even show People Tags in the Tags panel. 

But to answer your question with respect to Elements 11, if it is just one or two tags, you can right-click on one of the stacks and remove the stack.  You can then put the untagged photos through the analyzer again and place them in the correct stack. 

Or, if that doesn't work, add the names manually using the Mark Face button in single image view. 

Frankly, it has been so long since I have used Elements 11, I don't remember the details of how it operates, and I may be missing something.  But from what I have just seen, there have been many improvements over the past 10 years, both in terms of workflow and performance.